Following the practical development of electric bicycles and carts in recent years, transportation means serving as various kinds of moving bodies, such as electric motorcycles and electric motor vehicles, have begun to be rapidly powered by electricity. Specifically, as the replacement of motor vehicles powered by an internal combustion engine, there have been developed one after another vehicles such as hybrid vehicles powered by both gasoline engine and electric motor, electric vehicles powered by an electric motor driven by an onboard battery that is charged by a household-use power source or by a battery charger installed at gas stations or power supply stations, and fuel cell vehicles that run while generating electricity by a fuel cell using hydrogen and the like as the fuel. The electric motorcycles, hybrid vehicles and electric vehicles have already been put into practical use, and have begun rapidly becoming popular in the domestic market as well.
Since gasoline vehicles, diesel vehicles, motorcycles, etc. powered by the conventional internal combustion engine generate not only exhaust sound and engine sound which their power sources emit but also road noise in running, pedestrians, cyclists, etc. going around town streets can recognize those vehicles approaching by their engine and exhaust sound and the like. However, when running at low speed, the hybrid vehicles do not run powered by the combustion engine, but they are mainly in a running mode powered by the electric motor; therefore, no engine sound nor exhaust sound is generated, and furthermore, as for the electric vehicles and fuel cell vehicles, they run driven by the electric motor over the whole range of driving, which has made them extremely quiet electric moving bodies. However, pedestrians, cyclists, etc. who are in the vicinity of such quiet electric moving bodies as above cannot recognize by sound the approaching of the electric moving bodies, such as hybrid vehicles, electric vehicles and fuel cell vehicles, that run driven by the electric motor that generates little sound and is quiet, which therefore might cause occurrence of accidental contact of the pedestrians, etc. with those quiet electric moving bodies.
Therefore, in order to aim at resolving the foregoing problem in that quietness, which should be the advantage inherent to the hybrid vehicles, electric vehicles and fuel cell vehicles, sometimes become harmful, there have been proposed various systems other than klaxons mounted on conventional vehicles and operating following drivers' intention, that operate independently of the drivers' intention so as to give an alarm about the presence of the vehicles of their own.
For example, an electric vehicle is disclosed in Patent Document 1, which includes a running status detection means that detects and outputs a running status of the vehicle, an alarm sound generation means that emits sound to the outside based on the detected running status, and a control means that takes control of driving the alarm sound generation means, and can emit alarm sound, engine sound and the like so as to inform pedestrians around the vehicle of its approaching, and in which a klaxon and speaker are utilized as the alarm sound generator.
In addition, it is disclosed in Patent Document 2 that a vehicle, when running powered by its own electric motor, emits sound related to an audio signal so as to allow pedestrians to hear the sound and recognize the vehicle approaching. As a sound-emitting device for emitting the sound, there is disclosed a one that utilizes a speaker array made up of general cone speakers (moving magnet type).
Moreover, a piezoelectric speaker is disclosed in Patent Document 3 as a speaker system. This piezoelectric speaker is a high-impedance piezoelectric speaker in which electrodes are formed on both sides of a piezoelectric (piezoelectric element) and a sound signal is applied to the piezoelectric and which incorporates as the basic constituents a frame having a mounting flange, an elastic diaphragm, a piezoelectric diaphragm and a discoid damper. It is also disclosed that not only flat frequency characteristics can be obtained by this speaker system, but also its lower frequency limit and sound-pressure frequency characteristics can be enhanced by configuring a mechanical vibration system, so that a speaker system that also excels in performance can be provided, and so forth.
On the other hand, a vibration actuator that is mounted in mobile communications devices such as a pager (pocket bell) and mobile telephone and provided with a function of generating vibration is disclosed in Patent Document 4. In this vibration actuator, a coil is disposed in an air gap in a magnetic circuit including a permanent magnet that is fixed to its center column retaining an air gap between the damper and itself. In Patent Document 4, it is disclosed that the magnetic circuit is disposed concentrically with respect to the coil so that the coil is movably supported, the center column is provided with a stopper portion extending perpendicularly to axial directions and contacts to the damper, this stopper portion is formed stepwise and contacts to the damper, and the cross-section of which is shaped in an arc.
Moreover, in Patent Document 5 is shown a mounting structure of a panel-type speaker that vibrates the panel of a mobile telephone so as to use it as a speaker. In Patent Document 5, it is disclosed that an exciter is attached to the panel, and the panel and the casing are supported by a suspension in which a continuously-foamed material is sandwiched by a thin film, such as PET, and acrylic glue and the like. It is also disclosed that the space between the panel and casing is sealed with a waterproof film in this structure, and moisture is thereby prevented from entering there. Furthermore, it is also described that since the continuously-foamed material is used as the elastic material, sound pressure as the speaker is not lowered.